The present invention relates to an improvement in electro-optical display devices utilizing polarized light.
Those skilled in the art will be aware that unlike display devices which themselves generate light, such as cathode ray tubes, discharge tubes, electroluminescent diodes and so on, electro-optical display devices operate by modulating transmitted or reflected light emanating from an external source.
The majority of electro-optical devices utilizes polarized light. For many of them, this is a mandatory condition since it is linked with their principle of operation; such is the case with devices in which a plate of material is subjected to an electric field, producing variation either in the double refraction property of the plate (K.D.P., nematic liquid crystals), or in its rotating power (cholesteric liquid crystals or nematic liquid crystals exhibiting a twisted structure). In other devices, such as liquid crystals utilizing the dynamic scattering effect, polarized light may be used simply to improve contrast. Whatever the case, the electro-optical plate is illuminated by linearly polarized light, either from a laser source, or more generally from a conventional natural light source associated with a polarizer; the light coming from the plate reaches the observer's eye, after passing through a polarizer doing duty as analyser.
In the majority of these devices, the modulator has to be illuminated with parallel light and this limits the angle within which it is visible. It is possible to increase the viewing angle by arranging a diffuser between the observer and the exit polarizer; however, there is then a loss of contrast which is the greater the higher the ambient lighting level, since the observer's eye then receives, superimposed upon the light coming from the modulator, a substantial part of the ambient light which is scattered by the diffuser.